Diamine compounds are important chemical raw materials, which may react with dianhydride to form polyimide (PI), or react with diacid to form a polyamide (PA) such as nylon, or react with carbonyl dichloride to form diisocyanate. The diisocyanate may further react with polyol to form polyurethane (PU). Bis(para-aminocyclohexyl) methane (PACM) is an important cycloaliphatic diamine, and its raw material includes 4,4′-methylenedianiline (MDA), wherein the benzene rings of the MDA can be completely hydrogenated to form the PACM. The hydrogenated product (PACM) includes three isomers: (cis,cis)-isomer (also known as (c,c)-isomer), (cis,trans)-isomer (also known as (c,t)-isomer), and (trans,trans)-isomer (also known as (t,t)-isomer). The molar ratio of the (t,t)-isomer in the PACM will influence the physical properties of the PACM. If the molar ratio of the (t,t)-isomer in the PACM is close to 50 mol % (balance composition), the PACM will be PACM-50. The PACM-50 has a high melting point, and is solid at room temperature under normal pressure. If the molar ratio of the (t,t)-isomer in the PACM is close to 20 mol % (balance composition), the PACM will be PACM-20. The PACM-20 has a low melting point, thereby being a liquid at room temperature. The major application of the PACM in early age is preparing nylon, such that high molar ratio of the (t,t)-isomer is beneficial in preparing transparent nylon. In other words, the PACM-50 is the mainstream of the production. However, the PACM-20 liquid has become mainstream in recent years because PACM-20 can be used to prepare diisocyanate liquid for the rapidly developing PU industry.
The diisocyanate prepared from the PACM raw material has excellent optical stability (anti-yellowing property), climate resistance, and mechanical properties, and is therefore suitable for preparing transparent polyamide resin, curing agent of epoxy resin, strong electrical insulator, raw material of magnetic binder, intermediate of dye, or medicine. Diisocyanate is widely applied in the optical, electronic, and medical fields, and the market for it is continuously increasing.
The hydrogenation of MDA for synthesizing PACM is developed and has been reported on many references, many years ago. Not only the reactivity and selectivity in the hydrogenation need to be included in consideration, but also the molar ratio of the isomer should be controlled. The design and development of the catalyst is critical in the hydrogenation of MDA. In recent years, patents have been continuously granted for novel catalysts. This means that the hydrogenation catalyst still needs to be modified. For example, a PACM product with a low (t,t)-isomer molar ratio prepared by hydrogenating the MDA at a high conversion rate is still called for.